The aircraft was on a daily scheduled passenger service between Vienna and Dublin. The aircraft commander, who was the non-handling pilot, was suddenly seen by the co-pilot to be leaning heavily against the flight controls and apparently unconscious.The co-pilot immediately took control of the aircraft and declared a medical emergency to Dublin ATC and requested a priority landing and medical assistance on arrival.The commander regained consciousness at about this time and he was removed by ambulance to Beaumount Hospital, Dublin, for medical examination.He was discharged later that afternoon following various medical tests.